The invention relates generally to the field of solid state lasers and more specifically to improved methods and devices for converting a pump laser beam to an output beam of a desired wavelength.
A wavelength conversion device enables standard single wavelength lasers to perform multiple functions. One application of a wavelength conversion device is to use a common medical laser as a pump laser and to produce output wavelengths in the 570-850 nm range. Such a device can be used to facilitate a variety of medical therapies, such as in ophthalmology for retinal photocoagulation, illumination of photodynamic therapy (xe2x80x9cPDTxe2x80x9d) drugs for the treatment of Age Related Macular Degeneration (xe2x80x9cAMDxe2x80x9d) and other disorders, and transpupilary thermo therapy (xe2x80x9cTTTxe2x80x9d).
When a laser is used to treat diseases of the eye, the laser""s power and output wavelength(s) need to be carefully controlled. The wavelength of a laser must be matched to the absorption of a particular PDT drug to stimulate its effect. Ophthalmic laser therapies require a variety of visible and near-infrared wavelengths, depending in part on the desired depth of beam penetration. Ophthalmic PDT treatment of AMD requires about 300 mW of continuous wave laser power.
PDT drugs are typically illuminated with semiconductor diode lasers which output beams with wavelengths in the range of 630-810 nm. A drawback to the use of semiconductor diode lasers for illumination of PDT drugs is that a given diode laser is limited in the wavelength range it can produce. Typically, a single diode laser can provide on the order of 5 nm of wavelength range. Therefore, many diode laser devices would be required to illuminate all possible PDT drugs.
Laser pumped dye lasers can readily provide tunable output beams ranging from blue-green wavelengths in the visible range to near-infrared wavelengths. Laser pumped dye lasers can also be used to create typical PDT wavelengths, but are more typically used in non-ophthalmic PDT applications where an output beam with a higher power is desired. Argon ion lasers are used to optically pump an organic dye liquid in either a continuous or a rapidly pulsed mode of operation. Ultraviolet or green solid state lasers are also used to pump liquid dye lasers, typically in a pulsed mode of operation. Due to rapid decomposition of the dye, thermal-optical effects and triplet state absorption, these dye lasers have fluid circulation systems to refresh, filter and rapidly flow the dye through the lasing area. In many cases, these organic laser dyes are dissolved in solvents, which results in operational and safety problems.
The operational problems associated with liquid dye lasers have spurred research into the development of solid state dye lasers, which can be made by impregnating a variety of materials with laser dye. A solid state laser made by impregnating laser dye into a polymer can produce an optical performance comparable to that of a liquid dye laser. (See, e.g., R. Hermes, T. Allik, S. Chandra, J. Hutchinson, High Efficiency Pyrromethene Doped Solid State Dye Lasers, in Appl. Phys. Lett. 63(7), p. 877 (Aug. 16, 1993).) Unfortunately, results to date show that there are significant limitations to the useful lifetime of dye impregnated solid material and severe thermal optical aberrations in the solid dye laser material. (See, e.g., T. Allik, S. Chandra, T. Robinson, J. Hutchinson, G. Sathyamoorthi and J. Boyer, Laser Performance and Material Properties of a High Temperature Plastic Doped With Pyrromethene-BF2-Dyes, in Mat. Res. Soc. Symp. Proc. Vol. 329, p.291 (1994).)
Techniques such as moving the laser media have been used with solid state crystalline or glass lasers to reduce the average heat loading in the media, thereby reducing thermally induced optical aberrations. (See U.S. Pat. No. 4,890,289, Fiber Coupled Diode Pumped Moving Slab Laser.) The technique of fabricating the laser medium into a dye impregnated plastic rod and slowly rotating the rod has been proposed. This technique decreased the thermally induced optical aberrations at low pulse repetition rates. (S. Chandra, T. Allik, A. Floener, Compact, High Brightness, Solid State Dye Laser, in OSA Proc. on Adv. Solid-State Lasers, Vol. 24 (B.H.T. Chai and S. A. Payne, eds.), Optical Society of America, 1995.) An example of a rotating plastic-disk dye laser has been described. (A. Bank, D. Donskoy, and V. Nechitailo, High Average Power Quasi-CW Tunable Polymer Laser, in Proc. SPE Vol.2380, p. 292 (R. Scheps and M. Kokta, eds.), 1995). However, none of the foregoing examples has achieved more than limited success.
According to one embodiment of the present invention, an apparatus converts a pumping wavelength of a pump laser beam to a desired output wavelength. This apparatus includes: a solid medium impregnated with at least one type of laser dye; an input optical coating disposed on a first surface of the medium; and an output optical coating disposed on a second surface of the medium. The input optical coating and the output optical coating are configured to form an optical resonator within the solid medium. The output optical coating is partially reflective at the desired output wavelength and highly reflective at the pumping wavelength.
According to a second embodiment of the present invention, another apparatus converts a pumping wavelength of a pump laser beam to a desired output wavelength. The apparatus includes: a rotating device for rotating a disk-shaped solid medium impregnated with at least one type of laser dye; a resonator to resonate light from a pumped volume of the medium; a first optical coupling device for coupling a pump laser beam having a pumping wavelength to the medium; and a second optical coupling device for coupling a laser beam output from the pumped volume of the medium to an output device. The output laser beam has a wavelength different from the pumping frequency. Preferably, the rotating device rotates the medium at a rate fast enough to clear a volume of the medium heated by the pump laser beam in about 1 xcexcsec.
An input optical coating may be disposed on a first surface of the medium and an output optical coating maybe disposed on a second surface of the medium. The input optical coating and the output optical coating are configured to form an optical resonator within the solid medium. The input optical coating is transmissive (preferably with R less than 30%) at the pump wavelength and highly reflective (preferably with R greater than 98%) at the desired output wavelength. The output optical coating is partially reflective (typically 90-99%) at the desired output wavelength and may be reflective (for example, with R greater than 90%) at the pumping wavelength to enhance pump absorption.
If optical coatings are not disposed on the medium to form a resonator and external mirrors are used, antireflective coatings may be disposed on the medium.
The medium may be of any convenient shape, but in the preferred embodiment the medium is disk-shaped and has dimensions which approximate those of a conventional compact disk: about 120 mm in diameter and 1.2 mm thick. This allows the disk to be used in combination with many components of a conventional compact disk player, such as the platter, drive motor and disk changer. If the disk is made of plastic (for example, of dye-impregnated polymer), CD manufacturing molds and processes may be used. The ability to use existing components and manufacturing processes greatly reduces the manufacturing cost of the apparatus.
If the medium is larger than a conventional CD, it may be used for a longer time. Therefore, for some applications it is advantageous to form the medium into a size larger than that of a CD, despite the loss of some efficiencies associated with using a standard CD size.
In one embodiment, the device uses a modified version of the tracking servo found in a conventional compact disk player to move the pump laser beam in a radial direction while the disk is rotating. The tracking servo causes the pump laser beam to trace a spiral pattern on the disk.
Another embodiment causes a spiral pattern to be traced on the disk by moving the rotating disk laterally while the pump laser is fixed. This embodiment is advantageous if the input coupling device and the output coupling device are positioned on opposing sides of the medium.
In yet another embodiment, the medium is kept in a fixed position and the pump laser beam is moved to sweep the pump laser beam across the medium. A modified version of a scanning device, for example, can be used to actuate the pump laser beam. For this embodiment, the medium is preferably rectangular in shape, but may be formed into any convenient shape.
The solid medium can be impregnated with a plurality of laser dyes and can therefore emit laser light at a plurality of wavelengths. As described in more detail below, a user may choose to output a plurality of wavelengths, e.g., to simultaneously illuminate a xe2x80x9ccocktailxe2x80x9d of various PDT drugs. Alternatively, a user may isolate a particular wavelength which corresponds with, for example, a particular PDT drug or a particular type of laser therapy. A single medium may include a variety of optical coatings on different areas and each coating may correspond with an output wavelength. Multiple dyes can also be used when the output wavelength is very different from the pump wavelength. In this case, multiple dyes with overlapping emission and absorption wavelengths enable efficient stepwise transfer of excitation from the pump wavelength to the output wavelength.
The present invention also encompasses methods of treating eye diseases. One such method includes the steps of: providing at least one photodynamic therapy drug to a diseased portion of an eye; rotating a solid medium which is impregnated with at least one type of laser dye; illuminating a pumped spot on the solid medium with a pump laser beam having a pumping wavelength, thereby pumping a volume of the solid medium and causing the solid medium to emit an output laser beam which comprises a first output wavelength different from the pumping wavelength; and directing the output laser beam to a first photodynamic therapy drug in the diseased portion of the eye (typically via a fiber-optic cable from the laser to a slit lamp), thereby activating a first photodynamic therapy drug.
An alternative method of treating eye diseases includes the steps of: rotating a solid medium which is impregnated with at least one type of laser dye; illuminating a pumped spot on the solid medium with a pump laser beam having a pumping wavelength, thereby pumping a volume of the solid medium and causing the solid medium to emit an output laser beam which comprises a first output wavelength different from the pumping wavelength; and directing the output laser beam to a diseased portion of an eye, for photocoagulation or subcoagulation power for stimulation of tissue, as for TTT.
According to one variant of the first method, the first output wavelength is approximately 689 nm and the first photodynamic therapy drug is verteporfin, a PDT drug sold as Visudyne(trademark), a trademark of QLT PhotoTheraputics. The first output wavelength may be in the range of 630 nm to 810 nm. The pump laser beam may have a wavelength of approximately 520-532 nm.
According to one embodiment of the foregoing methods, the pumped spot can move in a spiral pattern on the rotating solid medium. A photodynamic therapy drug may be activated for the treatment of age-related macular degeneration. The pump laser beam may be a continuous wave laser beam having a pumping wavelength of approximately 532 nm. The pumped spot preferably moves on the solid medium such that the volume of the solid medium is refreshed approximately each xcexcsec.
The output laser beam may be used for transpupilary thermo therapy or for retinal photocoagulation. The solid medium may be impregnated with a plurality of laser dyes and the output laser beam may comprise a plurality of output wavelengths. More than one photodynamic therapy drug may be provided to the diseased portion of the eye and more than one photodynamic therapy drug may be activated.